Bond order simulation model: Coordination dependent bimetallic bonds

The variation of metal–metal bond strength or atomic site energy with number and type of atomic neighbors is described within a generalization of the bond order simulation (BOS) model, named the BOS-mixing model. The site energy for fixed coordination is expressed as a quadratic function of the number of existing mixed metal bonds. The three parameters in this functional form are (over)determined by the mixing energy as a function of composition for bulk bimetallic fcc systems. For the ten bimetallic alloys formed from Ni, Cu, Rh, Pd, and Ag, we find that the BOS-mixing model describes the composition dependence predicted by non-self-consistent electron density functional theory. Using the same BOS-mixing parameters obtained from this non-self-consistent electron density functional theory, the BOS-mixing model predicts accurately the microstructures (coordination arrangements) of clusters of Ni101Cu100, and Cu101Pd100 which are prototypical systems for bimetallic clusters. For Ni101Pd100 the BOS-mixing model displays some inaccuracies due to the atomic size mismatch of 10% which distorts the cluster shape from a perfect lattice structure. This is the same effect noted in previous applications of the BOS model and is thus expected.